NIRS is a non-invasive technique for the human brain cortex imaging based on the measurement of the NIR light emitted by suitable optical sources placed on the patient head and backdiffused to the surface after passing through the brain tissues. NIRS monitors the percentage of oxygenated and reduced hemoglobin in the blood, and it allows the real time functional imaging of the brain cortex also in tomographic mode (Diffuse Optical Tomography - DOT). The functional NIRS / DOT requires highly sensitive and fast photodetectors, and it can therefore be dramatically improved by using SiPMs, given their high optical responsivity, millions of times larger compared to that of conventional photodiodes. However, SiPMs present the problem of a reduced linearity range. We propose a new technique to overtake such issue and to allow the efficient implementation of SiPMs for the realization of high performance fNIRS / DOT systems.
Current fNIRS / DOT systems are based on either conventional photomultiplier tubes (PMTs), or silicon photodiodes, or avalanche photodetectors. PMTs require optical fibers for the signal transport from the patient head to the detector, with noticeable problems of optical coupling and due to the bulky instrument size. For the fNIRS / DOT systems with silicon photodiodes or avalanche detectors, the instrument size issues are solved, but such photodetectors present responsivities numerous orders of magnitude lower compared to SiPMs. Therefore the trade-off of sensitivity, size, and channel number of the current NIRS / DOT is much worse than what may be achievable by using SiPMs. The technology here proposed allows the optimal use of SiPMs in fNIRS / DOT, by exploiting at the best the SiPM linear region. Such approach opens the possibility of realizing multichannel systems with high spatial and temporal resolution, with sensitivities much higher compared to the state of the art.
Italy, PCT